Power and wavelength management for mixed-rate optical data systems

ABSTRACT

An optical transmission system exploits the reduced signal-to-noise (SNR) requirements for low-bit rate channels to devise a new wavelength channel allocation scheme which increases the number of channels that a WDM system can support. Wavelengths of low-bit rate channels are assigned outside a flat-gain window (i.e., flat-passband region) of the system. The channel allocation scheme uses the high-bit rate channels located in the flat-passband region of wavelengths and the lower-bit rate channels located outside this passband region with progressively lower-bit rate channels located farther outside this passband region. Low-bit rate channels are also assigned to region(s) of the passband where the non-linear threshold power level of the system may be exceeded.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to optically amplified lightwave systemsand, more particularly, to a method and apparatus for power andwavelength management for a mixed-rate optical data system.

BACKGROUND OF THE INVENTION

[0002] In optically amplified lightwave systems, system performance of agiven wavelength channel is determined, to first order, by its opticalsignal to-noise ratio (SNR). For higher bit rates larger SNRs(consequently higher signal powers) are required. (This can also beexplained by realizing that the number of photons/bit required by areceiver is, again to first order, roughly independent of the bit rate;consequently higher bit rates require higher signal powers.) Presentlyin wavelength-multiplexed (WDM) systems, the bit rates of all channelsare the same. Therefore the launch powers of all signals are nominallythe same.

[0003] Unfortunately under certain circumstances, the transmission ofall signal channels at the same data bit rate and at the same launchpower level can adversely affect system performance.

SUMMARY OF THE INVENTION

[0004] In accordance with the present invention, we exploit the reducedSNR requirements for low-bit rate channels to devise a new wavelengthchannel allocation scheme of a WDM system which increases the number ofchannels an optical transmission system can support. We assign thewavelengths of low-bit rate channels outside a flat-gain window (i.e.,flat-passband region) of the amplifiers. Moreover, our channelallocation scheme has the high-bit rate channels located in theflat-passband region of wavelengths and the lower-bit rate channelslocated outside this passband region with progressively lower-bit ratechannels located farther outside this passband region.

[0005] More particularly, in accordance with our invention, a wavelengthdivision multiplexed (WDM) system comprises (1) at least one opticalamplifier for amplifying optical wavelengths in at least two regions,each region exhibiting a different transmission gain characteristic forall wavelengths within that region; (2) at least one first-typetransmitter, each first-type transmitter transmitting an opticalwavelength selected from a first region of said at least two regions andmodulated at a data bit rate at or below a first data bit rate and (3)at least one second-type transmitter, each second-type transmittertransmitting an optical wavelength selected from a second region of saidat least two regions and modulated at a data bit rate at or below asecond data bit rate, where the second bit rate is lower than the firstbit rate.

[0006] According to one aspect of our invention, the first region is aflat-passband transmission region of the system having a gaincharacteristic which falls within a predetermined range and the secondregion has a gain characteristic outside that predetermined range.According to one embodiment, the second region is located in a roll-offregion of the system gain characteristic. In yet another embodiment,each second-type transmitter transmits at a wavelength within the secondregion the data bit rate which is selected so that the output powerlevel for said each wavelength in the second region does not exceed thenon-linear threshold level of the transmission fiber of the system.

[0007] The method of our invention comprises the steps of (1)determining the transmitted power versus wavelength characteristics ofthe system for each wavelength used in a WDM system; (2) for at leastone wavelength having a transmitted power level within a predeterminedrange, selecting a transmission data bit rate at or below a first databit rate; and (3) for at least one wavelength having a transmitted powerlevel outside said predetermined range, selecting a transmission databit rate at or below a second data bit rate, said second data bit ratebeing lower than the first data bit rate.

BRIEF DESCRIPTION OF THE DRAWING

[0008] In the drawings,

[0009]FIG. 1 shows an illustrative block diagram of a data transmissionsystem useful in describing the operation of the present invention;

[0010]FIG. 2 shows, illustratively, the launch power level for each ofthe optical wavelength signals being transmitted over the system;

[0011]FIG. 3 shows, illustratively, the output power level of each ofthe optical signals at the receiver location of the system;

[0012]FIG. 4 shows, in accordance with the present invention, thatcertain wavelength signals of FIG. 3 should be transmitted at reduceddata bit to improve their performance in the presence of the existingnoise levels; and

[0013]FIG. 5 shows, in accordance with the present invention, whichwavelength signals should be transmitted at reduced data bit and outputpower levels so that they do not exceed the non-linearity threshold ofthe of the transmission fiber of the system.

[0014] In the following description, identical element designations indifferent figures represent identical elements.

DETAILED DESCRIPTION

[0015]FIG. 1 shows an illustrative block diagram of a wavelengthdivision multiplexed (WDM) data transmission system in which the methodand apparatus of the present invention may be utilized. As shown, thetransmitter location 100 includes N transmitters 101 connected through awavelength multiplexer 102 and an optical amplifier 103. Each of thetransmitters 101 transmit at a different wavelengths λ1-λN and aremodulated at a data bit rate. The output 104 from the transmitterlocation 100 is transmitted over M of optical spans, 1-M, to a receiverlocation 110. Each optical span, 1-M, includes an optical fiber sectionand an optical amplifier OA. Each span 1-M has essentially zeroloss/gain, the amplifier OA providing optical gain to compensate for theoptical loss in the fiber section. The receiver location 110 includes awavelength demultiplexer 105 which demultiplexes the receivedwavelengths.

[0016] Shown in FIG. 2, is the signal at the output 104 of thetransmitter location 100. In the prior art, each of the N transmitters101 transmit at different wavelengths λ1-λN, but each of thetransmitters 101 was modulated to transmit at the same data bit rate andat the same power level.

[0017]FIG. 3 shows, illustratively, the output power level of each ofthe optical signals received at the input 106 at receiver location 110,as measured using an optical spectrum analyzer (OSA) 120 of FIG. 1. Theshape of the power spectrum 301 of the received signal is the result ofthe cascaded gain versus wavelength characteristics of the system ofFIG. 1. Each of the amplifiers OA exhibit, to one degree or another,“gain shaping ” which when cascaded together provides a wavelengthwindow (hereinafter a passband region) of operability shown in FIG. 3.That is, as a result of this amplifier gain shaping, the received powerspectrum 301 generally has a passband-type characteristic where thepower level in a central passband region 302 is relatively flat (i.e.,having a gain within a predetermined dB range) and the power spectrumrolls-off at a significant rate in the region below, 303, and in theregion above, 304, the flat-passband region 302. Normally, in prior artWDM systems with all wavelength channels operating at the same bitrates, channels located within the passband region 302 will generally beoperating at substantially the same signal-to-noise ratio (SNR). The SNRfor a particular wavelength channel is determined by the ratio of thesignal, e.g., 305, to the noise level in that channel, e.g., 306. Thenoise is generally the result of the amplified spontaneous emission(ASE) noise generated in the amplifiers OA. Since the ASE noise isbroadband, it has a spectrum that essentially tracks the amplifier powerspectrum 301. The noise level spectrum is such that at wavelengthsbeyond the edges of the passband 301 (i.e., roll-off regions 303 and304), the noise level has not fallen-off as fast as the signal leveland, as a result, the SNR has deteriorated for wavelengths in theregions below 303 and above 304 the passband 302. Consequently, in priorart WDM systems they have either avoided using these wavelength channelslocated outside the passband region 302 or they have suffered adeterioration of performance at these wavelength channels due tooperation at a reduced SNR.

[0018] In accordance with the present invention, we have determined howto improve the transmission performance at wavelength channels in theregions below 303 and above 304 the passband 301. Since low-bit ratesignals tolerate lower SNRs, we propose locating low-bit rate wavelengthchannels in the regions 303 and 304 outside the passband region 301 offlat gain. Our wavelength channel allocation scheme uses high-data bitrate channels in the passband region 301 and lower-data bit ratechannels located outside this passband region 301 (i.e., in regions 302and 303). Our technique also uses progressively lower-bit rate channelsfor wavelengths that are farther outside this passband region 301.

[0019]FIG. 4 shows, in accordance with the present invention, that thewavelength channels 307 and 308 in regions 303 and 304, respectively, ofFIG. 3 located outside this passband region 301 are transmitted atreduced data bit rates (compared to the data bit rates of thewavelengths in passband range 302) to compensate for their reduce signallevels. Since reliable transmission at a lower bit rate reduces the SNRrequired for reliable transmission, a reduced bit rate can be found toimprove performance for wavelength channels 307 and 308 in the presenceof the existing noise levels. It should be noted that while the presentinvention limits the maximum data rate in the roll-off regions 303 and304 to be less than the maximum data rate in passband region 302,obviously the data rates used by a wavelength channel in any of theseregions could always be less than the maximum rate for that region. Ourtechnique also enables additional wavelength channels in regions 403 and404, also operating at a reduced bit rate, to be satisfactorily operatedin the regions 303 and 304. In prior systems, the wavelength channels inregions 403 and 404 were never utilized because of their poor SNR (atthe higher data bit rates). As noted, to obtain satisfactory performancethe wavelength channels in regions 403 and 404 will have a maximum databit rate that decreases proportionally with decreases in the powerspectrum 301. That is, the farther a wavelength is located away from thepassband 301, the lower will be the maximum data bit transmission ratefor that wavelength. In our example in FIG. 4, the maximum data bit ratefor wavelength 405 (407) is lower than for wavelength 406 (408) and themaximum data bit rate wavelength 406 (408) is lower than for wavelength307 (308).

[0020] Thus our invention contemplates optical transmission systems inwhich the data bit rates of wavelength channels multiplexed on the samefiber are not all the same. For example, there could be a mixture of thestandard transmission rates from OC-192 (10 Gb/s) to OC-3 (155 Mb/s) orlower transmitted on the same fiber. Obviously, the techniques of thepresent invention is not limited to the use of these standardtransmission rates but can use any bit rates.

[0021]Fig. 5 shows, another aspect of the present invention, whereselected wavelength channels are operated at a reduced data bit andoutput power levels since when they would otherwise have exceeded thenon-linearity threshold of the system amplifiers. The illustrative powerspectrum 501 shows an optical system having a passband region 502 thatincludes a region 505 where the non-linearity threshold 510 has beenexceeded. The non-linearity threshold 510 is the power level above whichthe optical fibers of the system operate in a non-linear manner.Operating the optical fibers in this non-linear region would have adeleterious affect on system performance. Thus, in our example, thepower level of wavelengths 521 and 522 cause non-linear operation in thesystem fibers. We have recognized that if the signal levels of channels521 and 522 is reduced below the non-linear threshold level 510, theoperating data bit rates of channels 521 and 522 could be reducedproportionally to a lower data bit rate which would maintaining thedesired level of system performance at this lower data bit rate. Thisoccurs as previously discussed, because lower bit rate signals canoperate satisfactorily at a lower SNR level than higher bit ratesignals. Thus, we select the new reduced signal level 511 for channels521 and 522 so that it would not exceed the non-linearity threshold 510and then select a bit rate at that reduced signal level that results insatisfactory system data transmission performance for channels 521 and522 (e.g., the desired system error rate performance).

[0022] The mixed bit rate data transmission of the present invention isdesired in situations where (1) the SNR is poor for some wavelengthchannels and (2) power needs to reduced for some wavelength channels toavoid non-linear operation.

[0023] Significant cost savings can be realized by purposely usinglower-power transmitters for lower-bit rate channels. Not only are suchtransmitters cheaper but fiber nonlinear effects are reduced if lessoptical power is launched. Also, systems limited by total amplifieroutput power can accommodate more channels. (Technology and cost placeconstraints on the total amplifier power. Also, safety concerns placelimits on the total power.) With this strategy some care needs to betaken in wavelength allocation. Low-power (low bit rate) channelsprobably should not be interspersed among high-power (high bit rate)channels because of potential filtering problems. A neighboring highpower channel will interfere with a low-power channel because of finitefilter rejection. Therefore in such situations, low-power channels couldbe “banded” together (e.g., 303 and 304 of FIG. 3 and 503 of FIG. 5) andhigh-power channels could be in separate wavelength bands (e.g., 302 ofFIG. 3). Moreover, our channel allocation scheme enables the high-bitrate channels to be utilized in the passband region (302 of FIG. 3) andthe lower-bit rate channels to be utilized regions (e.g., 303 and 304 ofFIG. 3) outside this passband region, with progressively lower-bit ratechannels located farther outside this passband region. Additionally,lower bit rate channels can also be utilized and banded together inregions (e.g., 505 of FIG. 5) where the non-linear threshold may beexceeded. Thus, if a plurality of bit rates are used by the wavelengthchannels of the passband region 302, the wavelength channels closest tothe roll-off regions 303 and 304 and peak-gain region 505 should use thelowest bit rates to minimize interference with the wavelength channelsin regions 303, 304, or 505. Similarly, if a plurality of bit rates areused by the wavelength channels of the roll-off regions 303 and 304 andpeak-gain region 505, the wavelength channels closest to the passbandregion 302 should use the highest bit rates to minimize interferencewith the wavelength channels in the passband region 302.

[0024] What has been described is merely illustrative of the applicationof the principles of the present invention. Other methods andarrangements can be implemented by those skilled in the art withoutdeparting from the spirit and scope of the present invention.

What is claimed is
 1. A wavelength division multiplexed (WDM) systemcomprising at least one optical amplifier for amplifying opticalwavelengths in at least two wavelength regions, each region exhibiting adifferent transmission gain characteristic for all wavelengths withinthat region; at least one first-type transmitter, each first-typetransmitter transmitting an optical wavelength selected from a firstregion of said at least two regions and modulated at a data bit rate ator below a first data bit rate and at least one second-type transmitter,each second-type transmitter transmitting an optical wavelength selectedfrom a second region of said at least two regions and modulated at adata bit rate at or below a second data bit rate, where the second bitrate is lower than the first bit rate.
 2. The system of claim 1 whereinthe first region is a flat-passband transmission region of the systemhaving a gain characteristic which falls within a predetermined rangeand the second region has a gain characteristic outside thatpredetermined range.
 3. The system of claim 2 wherein for eachsecond-type transmitter transmitting at a wavelength within the secondregion the data bit rate is selected so that the output power level forsaid each wavelength in the second region does not exceed the non-linearthreshold level of a transmission optical fiber used in the system. 4.The system of claim 2 wherein the second region is located in a roll-offregion of the system gain characteristic.
 5. The system of claim 2including a group of second-type transmitters each transmitting at adifferent data bit rate, wherein a second-type transmitter transmittingat a wavelength which is closest to the first region is selected totransmit at a higher data bit rate than other second-type transmittersof said group.
 6. The system of claim 2 including a group of second-typetransmitters each transmitting at a different data bit rate, wherein thedata rate of all of the second-type transmitters transmitting areselected so that the data rate of second-type transmitters which have awavelength which is closest to the first region are selected to transmitat a higher data bit rate than other second-type transmitters of saidgroup which have wavelengths that are farther from the first region. 7.The system of claim 2 including a group of first-type transmitters eachtransmitting at a different data bit rate, wherein the data rate of allof the first-type transmitters transmitting are selected so that thedata rate of first-type transmitters which have a wavelength which isclosest to the second region are selected to transmit at a lower databit rate than other first-type transmitters of said group which havewavelengths that are farther from the second region.
 8. A method ofoperating a wavelength division multiplexed (WDM) system comprising thesteps of determining the transmitted power versus wavelengthcharacteristics of the system for each wavelength used in the system;for at least one wavelength having a transmitted power level within apredetermined range, selecting a transmission data bit rate at or belowa first data bit rate; and for at least one wavelength having atransmitted power level outside said predetermined range, selecting atransmission data bit rate at or below a second data bit rate, saidsecond data bit rate being lower than the first data bit rate.
 9. Themethod of claim 8 wherein for each wavelength outside said predeterminedrange, the data bit rate is selected so that the output power level forsaid each wavelength does not exceed the non-linear threshold level oftransmission optical fiber used in the system.
 10. The method of claim 8wherein each wavelength located outside said predetermined range is in aroll-off region of the transmitted power versus wavelengthcharacteristics of the system.
 10. The method of claim 8 including agroup of at least two wavelengths located outside said predeterminedrange, each wavelength of said group being modulated at a different databit rate and wherein a wavelength in said group which is closest to thepredetermined range is selected to transmit at a higher data bit ratethan other wavelength located in said group.
 11. The method of claim 8including a plurality of wavelengths located within said predeterminedrange, each wavelength of said plurality being modulated at a differentdata bit rate and wherein a wavelength in said plurality which isclosest to a wavelength located outside said predetermined range isselected to transmit at a lower data bit rate than other wavelengthlocated in said plurality.